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.TH SEM_WAIT 3C "Feb 5, 2008"
.SH NAME
sem_wait, sem_trywait \- acquire or wait for a semaphore
.SH SYNOPSIS
.LP
.nf
#include <semaphore.h>

\fBint\fR \fBsem_wait\fR(\fBsem_t *\fR\fIsem\fR);
.fi

.LP
.nf
\fBint\fR \fBsem_trywait\fR(\fBsem_t *\fR\fIsem\fR);
.fi

.SH DESCRIPTION
.sp
.LP
The \fBsem_wait()\fR function locks the semaphore referenced by \fIsem\fR by
performing a semaphore lock operation on that semaphore. If the semaphore value
is currently zero, then the calling thread  will not return from the call to
\fBsem_wait()\fR until it either locks the semaphore or the call is interrupted
by a signal. The \fBsem_trywait()\fR function locks the semaphore referenced by
\fIsem\fR only if the semaphore is currently not locked; that is, if the
semaphore value is currently positive. Otherwise, it does not lock the
semaphore.
.sp
.LP
Upon successful return, the state of the semaphore is locked and remains locked
until the \fBsem_post\fR(3C) function is executed and returns successfully.
.sp
.LP
The \fBsem_wait()\fR function is interruptible by the delivery of a signal.
.SH RETURN VALUES
.sp
.LP
The \fBsem_wait()\fR and \fBsem_trywait()\fR functions return  \fB0\fR if the
calling process successfully performed the semaphore lock operation on the
semaphore designated by \fIsem\fR. If the call was unsuccessful, the state of
the semaphore is unchanged, and the function returns \fB\(mi1\fR and sets
\fBerrno\fR to indicate the error.
.SH ERRORS
.sp
.LP
The  \fBsem_wait()\fR and \fBsem_trywait()\fR functions will fail if:
.sp
.ne 2
.na
\fB\fBEINVAL\fR \fR
.ad
.RS 11n
The \fIsem\fR function does not refer to a valid semaphore.
.RE

.sp
.ne 2
.na
\fB\fBENOSYS\fR \fR
.ad
.RS 11n
The \fBsem_wait()\fR and \fBsem_trywait()\fR functions are not supported by the
system.
.RE

.sp
.LP
The  \fBsem_trywait()\fR function will fail if:
.sp
.ne 2
.na
\fB\fBEAGAIN\fR \fR
.ad
.RS 11n
The semaphore was already locked, so it cannot be immediately locked by the
\fBsem_trywait()\fR operation.
.RE

.sp
.LP
The  \fBsem_wait()\fR and \fBsem_trywait()\fR functions may fail if:
.sp
.ne 2
.na
\fB\fBEDEADLK\fR \fR
.ad
.RS 12n
A deadlock condition was detected; that is, two separate processes are waiting
for an available resource to be released via a semaphore "held" by the other
process.
.RE

.sp
.ne 2
.na
\fB\fBEINTR\fR \fR
.ad
.RS 12n
A signal interrupted this function.
.RE

.SH USAGE
.sp
.LP
Realtime applications may encounter priority inversion when using semaphores.
The problem occurs when a high priority thread "locks" (that is, waits on) a
semaphore that is about to be "unlocked" (that is, posted) by a low priority
thread, but the low priority thread is preempted by a medium priority thread.
This scenario leads to priority inversion; a high priority thread is blocked by
lower priority threads for an unlimited period of time. During system design,
realtime programmers must take into account the possibility of this kind of
priority inversion. They can deal with it in a number of ways, such as by
having critical sections that are guarded by semaphores execute at a high
priority, so that a thread cannot be preempted while executing in its critical
section.
.SH EXAMPLES
.LP
\fBExample 1 \fRThe customer waiting-line in a bank may be analogous to the
synchronization scheme of a semaphore utilizing \fBsem_wait()\fR and
\fBsem_trywait()\fR:
.sp
.in +2
.nf
#include <errno.h>
#define TELLERS 10
sem_t  bank_line;      /* semaphore */
int banking_hours(), deposit_withdrawal;
void *customer(), do_business(), skip_banking_today();
thread_t tid;
\&...

sem_init(&bank_line,TRUE,TELLERS);  /* 10 tellers
                                       available */
while(banking_hours())
        thr_create(NULL, NULL, customer,
            (void *)deposit_withdrawal, THREAD_NEW_LWP, &tid);
\&...

void *
customer(deposit_withdrawal)
void *deposit_withdrawal;
{
    int this_customer, in_a_hurry = 50;
    this_customer = rand() % 100;
    if (this_customer == in_a_hurry)  {
        if (sem_trywait(&bank_line) != 0)
        if (errno == EAGAIN) {    /* no teller available */
            skip_banking_today(this_customer);
            return;
        }     /*else go immediately to available teller
               & decrement bank_line*/
    }
    else
        sem_wait(&bank_line); /* wait for next teller,
             then proceed, and decrement bank_line */
    do_business((int *)deposit_withdrawal);
    sem_getvalue(&bank_line,&num_tellers);
    sem_post(&bank_line); /* increment bank_line;
        this_customer's teller is now available */
}
.fi
.in -2

.SH ATTRIBUTES
.sp
.LP
See \fBattributes\fR(7) for descriptions of the following attributes:
.sp

.sp
.TS
box;
c | c
l | l .
ATTRIBUTE TYPE	ATTRIBUTE VALUE
_
Interface Stability	Committed
_
MT-Level	MT-Safe
_
Standard	See \fBstandards\fR(7).
.TE

.SH SEE ALSO
.sp
.LP
.BR sem_post (3C),
.BR attributes (7),
.BR standards (7)
